feat(data-testing): add throws support to conformance cases

[krisnye]

Summary

• Case can now declare throws: true | "substring" instead of after/effects, asserting a transition/action rejects its input — honored uniformly by runSpec, runTransactions, and runActions via a shared expectThrows helper.
• Fixes discoverTransitions to recognize a data/state/ module whose first case is throws-only (previously only "after" in case was checked, so such a module would silently fail to be discovered as a transition).
• Docs: data-ai rules (state.md, conformance.md) now describe the throws case shape and its intended, narrow use (genuine precondition rejection — not the default "guard no-op, return {}" style).

Test plan

• pnpm -r run typecheck clean across the monorepo
• pnpm -r run test clean across the monorepo
• New unit tests for expectThrows (expect-throws.test.ts)
• New end-to-end fixture cases in run-spec.test.ts exercising throws: true and throws: "substring" through the real runSpec runner
• New type-level tests in cases.type-test.ts confirming throws cases type-check and after+throws together is rejected

🤖 Generated with Claude Code